1. Field of the Invention
The present invention relates to a hollow fiber separation membrane with an asymmetric structure that is obtained by partial carbonization of a polyimide hollow fiber separation membrane and exhibits a vastly increased gas separation performance. It further relates to a method, for efficient separation of halogen compound gases from gas mixtures, using the separation membrane.
2. Description of Related Art
High-permeability asymmetric gas separation membranes employing several types of polymer materials are known in the prior art and, recently, in Japanese Unexamined Patent Publication No. 60-179102 and Japanese Unexamined Patent Publication No. 1-221518, for example, there have been proposed processes for the production of porous carbon membranes, as gas separation membranes, with excellent chemical resistance wherein the organic polymer membranes are treated at high temperature, as well as carbon membranes (hollow fiber carbon membranes) obtained by those processes.
However, while Japanese Unexamined Patent Publication No. 60-179102 discloses a separation membrane with an asymmetric structure including both a carbonized membrane and a graphitized membrane composed substantially of carbon obtained by prolonged high-temperature heating of a membrane of polyacrylonitrile or the like, the gas permeation rate ratio of the carbon dioxide gas and nitrogen gas of the membrane has been very small. In addition, this publication does not mention a membrane obtained by partial carbonization of a polymer membrane.
Furthermore, Japanese Unexamined Patent Publication No. 1-221518 discloses hollow fiber carbon membrane fibers having numerous micropores of 3-5 xc3x85 in the middle portion, with an adsorption of no greater than 0.1 cm3/g for molecules of 6 xc3x85 or greater, and therefore it is completely different from a membrane having an asymmetric structure of which the major part consists of a portion with a porous structure having a pore size of 6 xc3x85 or greater.
On the other hand, Japanese Unexamined Patent Publications No. 4-119331 No. 4-193334 and No. 5-220360 each disclose a hollow fiber membrane with an asymmetric structure obtained by partial carbonization of an aromatic polyimide hollow fiber membrane with an asymmetric structure, and a process for its production.
Incidentally, halogen compounds, primarily SF6 (sulfur hexafluoride) gas, Freom(trademark) gas such-as fluorocarbon gas and chlorofluorocarbon gas, carbon tetrachloride and the like, are used as electrical insulating gases for gas-insulated electrical devices such as gas-insulated switchgear, gas circuit breakers, gas-insulated transformers, line air transmission devices, etc. Recently, as a measure against global warming gases that have become an issue on a global scale, it has been attempted to cut down on their usage by employing mixed gases comprising halogen compounds with other electrical insulating gases (carrier gases) as electrical insulating gases in gas-insulating electrical devices.
In gas-utilizing semiconductor processes in the semiconductor industry, there is a trend toward the exclusive use of perfluoro compounds such as CF4, C2F6, C3F8, C4F10, SF6 and NF3 for etching, rinsing and the like. These perfluoro compound gases are used in pure form, in a form diluted, for example, with air, nitrogen or another inert gas, or in the form of a mixture with another perfluoro compound gas or another carrier gas (for example, an inert gas).
Virtually all of these perfluoro compound gases contained in carrier gases are gases with an adverse effect on the environment, including global warming, and consequently they must be recovered and reutilized without being emitted into the atmosphere.
In Japanese Patent Application No. 10-365598, the present inventors have proposed a hollow fiber separation membrane having an asymmetric structure obtained by partial carbonization of an aromatic polyimide hollow fiber separation membrane with an asymmetric structure, for separation and recovery of these perfluoro compound gases, and have demonstrated that the perfluoro compound gases can thereby be efficiently separated and recovered.
However, since a higher efficiency of gas separation and recovery is preferred, and gas purity must be further increased for the reutilization of recovered gases, there is a demand for separation membranes with even higher separation performances.
It is an object of the present invention to provide a hollow fiber separation membrane with a very high permeation rate and high selectivity both for surpassing the conventional hollow fiber separation membranes with asymmetric structures obtained by partial carbonization of aromatic polyimide hollow fiber separation membranes with asymmetric structures. It is a further object to provide a process whereby halogen compounds can be separated and recovered from mixtures of halogen compounds and carrier gases (nitrogen, etc.) containing the aforementioned perfluoro compounds, at a much higher efficiency than by conventional processes.
These objects of the invention are attained by the present invention which has the following construction.
Specifically, the present invention provides a partially, carbonized asymmetric hollow fiber separation membrane which is a hollow fiber separation membrane having an asymmetric structure obtained by partial carbonization of an aromatic polyimide hollow fiber separation membrane with an asymmetric structure (precursor membrane), wherein
(1) the thickness of the aromatic polyimide hollow fiber membrane to be partially carbonized (precursor membrane) is 8-50 xcexcm,
(2) the thickness of the hollow fiber separation membrane obtained by partial carbonization is 8-45 xcexcm,
(3) the carbon content (wt %) of the hollow fiber separation membrane obtained by partial carbonization is at least 1.05 times the carbon content of the aromatic polyimide hollow fiber membrane to be partially carbonized (precursor membrane), and no greater than 90 wt %.
The present invention further provides a production process for a partially carbonized asymmetric hollow fiber separation membrane which is a process whereby an aromatic polyimide hollow fiber separation membrane with an asymmetric structure (precursor membrane) is partially carbonized to produce a partially carbonized hollow fiber separation membrane with an asymmetric structure, wherein
(1) the thickness of the aromatic polyimide hollow fiber membrane to be partially carbonized (precursor membrane) is 8-50 xcexcm,
(2) the thickness of the hollow fiber separation membrane obtained by partial carbonization is 8-45 xcexcm,
(3) the carbon content (wt %) of the hollow fiber separation membrane obtained by partial carbonization is at least 1.05 times the carbon content of the aromatic polyimide hollow fiber membrane to be partially carbonized (precursor membrane), and no greater than 90 wt %.
The present invention still further provides a method of separating halogen compound gases that comprises supplying a mixed gas containing at least one type of halogen compound gas and at least one type of carrier gas to the aforementioned partially carbonized asymmetric hollow fiber separation membrane, withdrawing gas comprising at least one type of carrier gas having a reduced halogen compound content from the permeating side of the membrane, and recovering gas with a greater concentration of at least one type of halogen compound from the non-permeating side of the membrane.